In oil well operations, it is often necessary to retrieve objects from deep well bores. Typically, the objects in the well bore are portions of drilling equipment such as drill rods having an internal and an external diameter. In the oil well drilling industry, the practice of retrieving such objects is often referred to as "fishing", and the object to be retrieved is referred to as a "fish".
One of the more common fishing techniques involves the use of a tapered tap fishing tool. The tapered tap fishing tool is normally a single three to four foot long section, with case hardened threads on the outside diameter of the section. The outside diameter increases from approximately one inch at a bottom nose to approximately four or five inches at the top. The tapered tap fishing tool is mounted on a bottom end of a rotatable down-hole drill rod and is put down the hole until the taper reaches the object to be retrieved. The taper enters the internal diameter of the fish and is rotated so that the case hardened, tapered threads tap the inside of the fish. The taper cuts threads into the softer material of the fish and engages the fish by screwing into the newly cut threads. When the fishing tool and the fish are so engaged, the fish can be withdrawn from the well bore.
Another technique for retrieving objects from well bores involves the use of box tap fishing tools. The box tap fishing tool differs from the tapered tap fishing tool in that the box tap fishing tool has a single section with a tapered, threaded internal diameter, with case hardened threads. The box tap fishing tool engages an exterior of the fish, unlike the tapered tap fishing tool, which is only capable of use on fish having internal diameters. Accordingly, the box tap fishing tool is a more versatile fishing tool than the tapered tap fishing tool.
Known box tap fishing tools tend to be shorter than tapered tap fishing tools and have a larger angle of taper than the tapered tap fishing tools. As a result, box tap fishing tools suffer the disadvantage of generally engaging fewer threads with the fish, and it is therefore more difficult to fish for heavier or more securely stuck fish with box tap fishing tools. One reason for the higher angle taper and the shorter length of the box tap fishing tools is the difficulty of boring and threading the internal diameter of the box tap fishing tools. Further, it is quite expensive to form tapered, threaded internal diameters of significant lengths. Because fishing tools are generally subjected to quite harsh use conditions, which frequently result in damage to the tools that renders them unusable, it is presently difficult to justify the expense of machining long internally threaded, tapered diameters in box tap fishing tools.
In view of the above-described disadvantages of known fishing tools, it is desirable to provide a fishing tool combining the advantages of the box tap fishing tool with the ability to engage a fish with a large number of threads of the tapered tap fishing tool. It is further desirable to provide a fishing tool that is easy to manufacture.
It is a further object to provide a fishing tool that is adaptable to a wider variety of fish diameters and can provide secure engagement with fish having a variety of different diameters. It is still a further object to provide a tool wherein, if damage to a portion of the tool occurs, replacement of the damaged portion is facilitated.
In accordance with one aspect of the present invention, an apparatus for retrieving objects from well bores is provided. The apparatus includes an uppermost segment having an internally threaded opening extending upwardly from a bottom end of the uppermost segment to a first location above the bottom end. One or more additional segments are provided, each of the additional segments having a top and a bottom end and being removably attachable, at the top end of each additional segment, to the bottom end of one of an upper one of the additional segments and the uppermost segment. Each of the additional segments has an internally threaded opening extending between the top end and the bottom end of the additional segment. Means are provided for removably securing a top end of the uppermost segment to a bottom end of a rotatable down-hole drill rod. The internally threaded openings of the uppermost and the additional segments are tapered from a first diameter at the first location in the uppermost segment to a second, larger diameter at a bottom end of a lowermost one of the additional segments.
In accordance with another aspect of the present invention, a method for retrieving an object from well bores is provided. According to the method, an uppermost segment of a retrieving apparatus is attached to a bottom end of a rotatable down-hole drill rod. The uppermost segment has an internally threaded opening extending upwardly from a bottom end of the uppermost segment to a first location above the bottom end. One or more additional segments of the retrieving apparatus are selectively attached to the uppermost segment. Each of the additional segments is removably attachable, at a top end of each additional segment, to a bottom end of one of an upper one of the additional segments and the uppermost segment. Each of the additional segments has an internally threaded opening extending between the top end and the bottom end of the additional segment. The retrieving apparatus is lowered down a well bore such that a portion of the object to be retrieved is received inside an internally threaded opening of the retrieving apparatus. The internally threaded opening of the retrieving apparatus is formed by the internally threaded openings of the uppermost and the additional segments. The object is engaged with threads of the internally threaded opening of the retrieving apparatus. The retrieving apparatus is rotated relative to the object such that threads of the internally threaded opening of the retrieving apparatus cut into the object and secure the object in the internally threaded opening of the retrieving apparatus. The retrieving apparatus in which the object is secured is removed from the well bore. The internally threaded openings of the uppermost and the additional segments are tapered from a first diameter at the first location in the uppermost segment to a second, larger diameter at a bottom end of a lowermost one of the additional segments.
In accordance with yet another aspect of the present invention, a method of making a tool for retrieving objects from a well bore is provided. According to the method, an internally threaded opening extending upwardly from a bottom end of an uppermost segment is formed in the uppermost segment. An internally threaded opening is also formed through each of one or more additional segments. The internally threaded opening in the additional segments extends from a top to a bottom end of each of the additional segments. The bottom end of the uppermost segment is secured to the top end of one of the additional segments. The internally threaded opening in the uppermost segment and the internally threaded opening in the additional segment are formed such that they are tapered to have a top diameter that is smaller than a bottom diameter.